When Yoritomo died in 1199 Tokimasa’s grandson Yoriie became the new shogun, although the greatest influence on the young man was actually Hiki Yoshikazu, Yoriie’s father-in-law and guardian. Yoriie was a difficult man, and developed a dislike for the Hôjô, at the same time damaging his own position through his youthful thoughtlessness. When a council was formed among the chief Bakufu retainers to moderate Yoriie’s judicial authority, Yoriie responded by turning on the man regarded as his most outspoken opponent, governor of Sagami Kajiwara Kagetoki. Although Kajiwara had been one of Yoritiomo’s most trusted retainers, he was hunted down and killed in Suruga by Bakufu troops in 1200. Tokimasa’s role in the downfall of Kajiwara is unclear, but his clan did benefit from the event by acquiring the now open province of Sagami. Kajiwara’s death also had the effect of tidying the political playing field in Kamakura, leaving the Hiki as the next major obstacle to Tokimasa’s ambitions. Knowing that there was little hope of supplanting Hiki at Yoriie’s side, Tokimasa placed his political capital on Minamoto Sanetomo, Yoriie’s younger brother. In 1203 Yoriie became very ill, and the Hôjô pushed through a plan that would divide the realm between Sanetomo and Yoriie’s son Ichiman. The Hiki Yoshikazu doubt guessed Tokimasa’s ploy, and plotted to have the Hôjô leader murdered. Tokimasa in turn caught wind of the danger he was in, and after consulting Ōe Hiromoto, made a preemptive strike. He invited Yoshikazu to his residence ostensibly for Buddhist services, and when Hiki unwisely came out Hôjô had the man murdered. Hôjô retainers and allies then descended on the Hiki residence and killed Ichiman and most of the Hiki’s important names, thus eliminating the Hiki in one decisive stroke. With his most powerful political supporter gone, Yoriie was isolated and within a few weeks stepped down from office. He went to live at the Shuzenji in izu and was murdered a year later-probably on the order of Tokimasa.

Tokimasa installed Sanemoto as the 3^rd Minamoto Shogun and began to exercise control through the Mandokoro, which he chaired along with Ōe Hiromoto. The most powerful man in Kamakura, Tokimasa was known as the shikken, a term that at the time referred to his position on the mandokoro. Later, shikken would come to refer to the role the Hôjô clan played in Bakufu politics - that of regents to a series of child-shoguns.

Ironically, Tokimasa may in the end have schemed once too often. In 1204, a certain Hiraga Tomomasa convinced Tokimasa that Hatakeyama Shigetada, Tokimasa’s son-in-law, was engaged in treasonous behavior. Tokimasa ordered his sons Yoshitoki and Tokifusa to kill Hatakeyama, over Yoshitoki’s objections. Hatakeyama was duly executed, but with a resentful Yoshitoki still convinced of his innocence. That Shigetada just so happened to be another potential rival to the Hôjô stranglehold on Kamakura politics likely had more to do with his death than any treasonous ideations.

Some time after the Hatakeyama affair, in 1205,Yoshitoki heard rumors that Tokimasa was planning to have Sanetomo assassinated and replaced with none other then Hiraga Tomomasa, a man of Minamoto stock. Yoshitoki and Masako, who also seems to have drawn away from her father, took steps to foil the scheme by putting Sanetomo under guard and killing Hiraga. Yoshitoki openly challenged his father’s authority, and left Tokimasa little choice but to step down. Tokimasa shaved his head and departed for exile in Izu province, his political career at an end. Whether or not Tokimasa really intended to have Sanetomo killed is anyone’s guess. Sanetomo was hardly a difficult character - most of his time was spent studying the arts - in particular poetry. Possibly Tokimasa had become a liability to Masako and Yoshitoki - we have no way of knowing. That Tokimasa’s career had a profound effect on the course of Japanese history is somewhat more obvious.